This is when you use a card with little or no Pillz so that the opponent will use a lot of Pillz against it and you will lose the fight but have a large Pillz advantage. It can also be so that the opponent uses a threatening or annoying card, but your bluff card had a Damage reducing or a SoA Ability.
It also works the other way around where you use a lot of Pillz on a card that you wouldn't normally use a lot of Pillz on E.G a damage reducer so that your opponent dosn't use many pillz an you win the fight. Common Bluffers are cards with heavy attack minipulation.
Bluffing is the main strategy of the game, as well as being able to call the opponent's bluff and judging if a risk is worth taking. Since there are only 12 Pillz spread over up to 4 rounds, there will rarely be a chance to simply overpower your opponent blindly and win the match.